Colorant dispersions of recording ink are used in various applications that utilize their colorant properties, such as in writing inks for pens or seal inks, in image forming apparatuses using liquid inks (e.g. wet type printers or inkjet printers) and so forth.
It is important that in these colorant dispersions the colorant should be uniformly dispersed at an intended particle diameter and the liquid property should be kept constant with time.
Furthermore, since a colorant dispersion is less likely to be used alone but is often used in combination with a resin or other additives depending on the purpose, the colorant dispersion needs not only to keep the property itself excellent but also to have no influence on the properties of the other additives.
Particularly when the colorant dispersion is used in a recording ink, as well as degradation with time of the colorant dispersion itself in the ink liquid, thickening of the ink liquid due to interaction with the other additives and adverse effects on degradation with time of the ink liquid and on printed images because of the combined use of the colorant dispersion with the other additives such as a resin in the recording ink, arise, which necessitates very careful selection of a dispersant for dispersing a colorant.
The ink incorporating such a colorant dispersion of recording ink is applied or transferred onto a medium and dried to form a membrane or a recorded image.
Recently, demand for aqueous inks is increasing in view of an environmental and safety perspective.
However, recording performance of the aqueous ink is easily affected by a medium to be used, which is responsible for various problems concerning the membrane or the image. Particularly when paper is used for the medium instead of film, the problems are exaggerated.
When the aqueous ink is used for recording on paper, substantial time is required for drying, and a fair amount of the aqueous ink permeates the paper due to excellent compatibility with the paper, which, particularly in the case of using uncoated, relatively nonsmooth paper, results in a low color density of the image formed by the colorant a fair amount of which has been lost by permeation into the paper. The latter problem has not been seen in recording using a solvent ink or a solventless ink.
In order to increase the color density, it is necessary to increase the amount of the colorant in the ink, which, however, not only disadvantageously adds to the cost, but also results in aggregation or precipitation of the colorant and in turn in bad liquid stability of the ink liquid because the colorant density is increased in the solvent. In addition to these problems, increasing the amount of the colorant leads to a problem of bad appearance caused by roughening surfaces of the image and the membrane.
Recently, methods for inkjet recording have become widespread as image forming methods, because they have advantages in that they facilitate full-color printing in the simper process than other method for recording and they allow for obtaining high-resolution images with a simply structured apparatus.
In the inkjet recording methods, a small amount of an ink is flown by applying a pressure produced by heat and so forth to the ink, the flown ink is allowed to adhere to a medium for image forming such as paper, and the adhered ink needs to be dried promptly (to permeate the medium for image forming) to thereby form images. Therefore, drying property or an image density associated with the use of the aqueous ink may be getting more and more important in the inkjet recording method.
Particularly recently, the inkjet recording methods are further developed in the field of industry and business, which increases demand for faster output of high density images.
For example, in Patent Literature 1, for achieving excellent dispersion stability of an aqueous ink or a high density of printed images, a method of incorporating a polymer produced by copolymerizing (meth)acrylic acid with a monomer having a hydroxyl group, etc. is proposed, and as an example of the monomer having a hydroxyl group a polyethylene glycol allyl ether is described.
However, the proposed aqueous ink fails to provide an adequate image density and has a drawback that when a colorant having a large specific surface area such as carbon black is dispersed the dispersion stability of the aqueous ink becomes poor with time.
In Patent Literature 2, for increasing resistance of an ink to film-forming stress, an ink binder produced by copolymerizing an alkyl (meth)acrylate and a monomer having a nitrogen atom and a monomer having a hydrophilic group is proposed, and as examples of the hydrophilic monomer a hydroxypolyethyleneglycol mono(meth)acrylate dialkyleneglycol allyl ether and a monomer having an acrylic acid sulfonic acid group are described. However, this ink also fails to provide an adequate image density.
In Patent Literature 3, an ink containing a polymer produced by copolymerizing sodium acrylate and a polyethyleneglycolmonoacrylate is proposed. However, this ink also fails to produce an adequate image density.
Furthermore in Patent Literature 4, a solventless resin produced by copolymerizing a polyalkyleneglycolacrylate (80%-100%) with another monomer (0%-20%) is proposed. However, the solventless resin has low water-solubility, therefore is not suitable for use in a colorant dispersion containing water, and also has little effect on improvement of an image density produced.    Patent Literature 1: Japanese Patent Application Laid-Open (JP-A) No. 2005-23284    Patent Literature 2: JP-A No. 2000-44861    Patent Literature 3: JP-A No. 09-183926    Patent Literature 4: JP-A No. 08-283560